The measurement of flow of a liquid which may be contaminated with foreign particles or gas bubbles presents problems. Transit time measurements provide accurate flow measurement of uncontaminated liquid. However, when the liquid is contaminated, Doppler measurement provides more reliable flow measurement. For example, water pumped from the ground during the production of methane gas from a coal seam or other underground shale varies from crystal clear to milky white with entrained methane. Many applications also have intermittent suspended particulates of coal, rocks or sand in the water. Dual mode meters are known, see Oldenziel et al. U.S. Pat. No. 5,533,408, Shekarriz et al. U.S. Pat. No. 6,067,861, Morgen et al. U.S. Pat. No. 6,871,148, and Ishikawa published application 2006/0174717.